Comment on attachment 8519864[details][review]
Github PR: https://github.com/mozilla-b2g/gaia/pull/25583
Looks good to me. In the last assertion, we check that strings representations of Blobs are not equals[1]. We could extract the data of the Blobs to check they are different. It sounds too overkill to do with Python though.
[1] {u'slice': None, u'type': u'image/png', u'size': 224580} != {u'slice': None, u'type': u'image/jpeg', u'size': 100045}